The present invention relates to an artificial vessel which has a compliance approximate to that of a vital vessel and a process for preparation of the same.
Many artificial vessels have been developed with the progress of vascular surgery. At present, examples of clinically used artificial arteries with a large diameter of not less than 6 mm are the DeBakey artificial vessel made of woven Dacron (USCI. Co., Ltd. of U.S.A.) and the Gore-Tex vessel (Gore. Co., Ltd. U.S.A.) which is made of expanded polytetrafluoroethylene (hereinafter referred to as "EPTFE")
In Sasajima et al, J. Artif. Organs, 12(1), 179-182 (1983), the compliances of the above artificial vessels were measured. The results are shown in Table 1.
TABLE 1 ______________________________________ Vessel Compliance ______________________________________ Thoracic aorta of dog 0.749 Abdominal aorta of dog 0.491 Carotid artery of dog 0.356 Double Velour Dacron 0.058 Woven Dacron 0.021 EPTFE 0.028 ______________________________________
As shown in Table 1, the compliances of the conventional artificial vessels are much smaller than those of vital vessels, which causes various problems due to compliance mismatch, such as anastomotic punnus hyperplasia, a long time after the grafting of the vessel in a living body. Particularly, the conventional artificial vessels cannot be clinically used as an artificial artery with a small diameter of not more than 6 mm because the compliance mismatch remarkably increases at such a small diameter, causing poor patency of the vessel. Therefore, the patient's own veins are used for vascular reconstructive surgery of coronary arteries or arteries below the knees.
In order to solve such compliance mismatch, a process for preparing an artificial vessel from an elastomer which has a porous wall and a compliance approximate to that of a vital vessel is disclosed in U.S. Pat. No. 4,173,689 to Lyman. This artificial vessel is prepared by immersing a mandrel into an elastomer solution, taking the mandrel out of the solution, coating the mandrel with the solution, and immersing it into a poor solvent such as water to deposit the elastomer on the mandrel. However, this process can only provide a vessel having very small pores on its wall and having a relatively dense structure. Although the compliance of the artificial vessel prepared according to the process disclosed in the U.S. Pat. is larger than that of a conventional artificial vessel, its compliance is still smaller than that of a vital vessel and is not sufficient. In addition, since it is difficult to coat the mandrel uniformly with the elastomer solution, an artificial vessel having the same properties in all of its parts cannot be prepared.
An artificial vessel for arterial grafting which comprises at least two porous elastomer zones and a solid elastomer zone is disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 150954/1982. As a result of the present inventor's study, it was found that an artificial vessel having the solid elastomer zone--porous elastomer zone--solid elastomer zone structure at its cross-section could not be prepared according to the procedures disclosed in the publication.
As mentioned above, in general the smaller the diameter of an artificial vessel for an artery becomes, the more it is important that the compliance of the vessel approximates that of a vital vessel. Although various efforts have been made to obtain an artificial vessel having a compliance approximate to that of a vital vessel, no artificial vessel having the required compliance has been realized.
Moreover, the preparations of the conventional artificial vessels are complicated, which makes the artificial vessels expensive.
In addition to compliance match, an artificial vessel requires properties such that the suture should not be readily frayed, the artificial vascular material can be optionally cut to a desired length, and no kinks are formed. When woven Dacron or woven polytetrafluoroethylene is used, a special textile technique is required in order for the vessel not to fray at the cut ends, and a special technique such as bellows processing is also required to avoid the formation of kinks, which makes the preparation of the vessel complicated and its price expensive. In the case of the EPTFE, a complicated preparation is required for stretching the PTFE, which makes the vessel expensive.
As a result of the inventor's continuous studies, it has been found that an inexpensive artificial vessel having a compliance approximate to that of a vital vessel can be prepared by extruding an elastomer solution from an annular nozzle and treating the inner and the outer sides of the tubular extrudate in a coagulating liquid. The artificial vessel obtained comprises a skin layer at least in the inner surface area of the vessel wall and a spongy structure in the inside region of the vessel wall.